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31.
《水文科学杂志》2013,58(3):473-487
Résumé Cette étude se situe au Niger en région sahélienne, près de la ville de Niamey, et a pour but la reconstitution des conditions de formation des différents types de nodules carbonatés que l'on trouve sur les anciennes terrasses du fleuve Niger. La chimie, la géochimie isotopique (radioactif et stable) et l'étude minéralogique des carbonates ont été utilisées pour dater les nodules et reconstituer les conditions de précipitation. Les résultats montrent bien l'authigénie et l'accrétion dans le premier mètre du sol des nodules de la vallée du Niger. L'âge de formation des nodules apparaît ancien, entre 1500 et 7000 ans BP. Cette plage de temps correspondrait à une période transitoire entre les conditions de sécheresse actuelles et l'importante phase humide régionale située entre 10 000 et 4500 ans BP. Cependant, la diversité des âges obtenus, ainsi que les différentes formes de nodules montrent que des facteurs très locaux (état de la nappe localement, végétation, texture du sol) ont dû aussi jouer un rôle dans le processus de précipitation des nodules. 相似文献
32.
Natural Magnetic Archives of Past Global Change 总被引：3，自引：0，他引：3
Rock magnetic parameters of sediments reflect palaeoenvironmental and palaeoclimatic conditions during deposition in the marine and in the continental realm. The processes which lead to enhancement or degradation of the magnetic properties are well known – though not completely understood. They include variable terrigenous material influx, organic and inorganic in situ formation of ferromagnetic minerals and diagenetic destruction of these minerals. Cyclical changes in the magnetic mineral assemblages occurring at the orbital periodicities involved in standard Milankovitch theory have been observed in numerous sedimentary records, at least during the last seven million years. This confirms the relationship between rock magnetism and past global change. Quantitative interpretation of the magnetic signals, however, is still in its infancy and is hampered by the complex interplay of the signal formation processes. 相似文献
33.
Taking varves to bits: Scanning electron microscopy in the study of laminated sediments and varves 总被引：2，自引：0，他引：2
Jean M. Dean Alan E.S. Kemp David Bull Jennifer Pike Gunilla Patterson Bernd Zolitschka 《Journal of Paleolimnology》1999,22(2):121-136
Conventional high resolution studies of varved sediments are able to identify clastic and biogenic laminae, but are often unable to resolve the nature of fine-scale lamination contained therein. This intra-annual signal provides us with the highest potential resolution from the sedimentary record and can be resolved using scanning electron microscopy (SEM). Six case studies from lacustrine and marine settings are presented to illustrate the combination of clastic and biogenic fabric types typically found in laminated sediments. Clastic laminae fabrics include those which originate through grain settling and those which are event deposits. The correct identification of event deposits is essential if varves are to be used chronologically. SEM-based biogenic laminae fabric studies have identified seasonal faunal successions where individual laminae may be less than 100 thick and most recently, deep chlorophyll maxima (DCM) summer diatom floras, providing an insight into seasonal scale processes. High resolution lamina fabric studies can provide a basis for generating records of seasonal and inter-annual variability, thus contributing to our understanding of lacustrine and marine processes and palaeoenvironmental interpretation. 相似文献
34.
Stuart A. Harris 《寒旱区科学》2010,2(5):0371-0383
In the Cordillera of western North America, the influence of the Pacific Interdecadal Oscillation only affects coastal areas west of the Coast Range and the lowlands of western and southern Alaska. The rest of the area is subject to a climate controlled by the relative strengths of three distinct air masses, viz., the cold cA/cP air that is dominant in winter, the mP air bringing cool moist air over the mountains throughout the year, and the dry hot cT air from the deserts of the southwestern United States. The Arctic Front marks the boundary between the cA/cP air mass and the other two. Changes in the relative strengths of these air masses appear to explain the climatic changes documented throughout the region. Thus, in the last 30 years, the average position of the Arctic Front has moved north from about 53°N to 58°N, causing the warming in northern British Columbia and cooling south of Calgary, Alberta. This concept of changing positions of the air masses also appears to explain the mechanism behind the past climatic changes in this region. During the last Neoglacial event （c.1400-1900 A.D.）, it appears that the cA/cP air mass had strengthened enough to push the Arctic Front south of the 49th parallel. Incursions of mP air increased with localized areas of short-term heavy snowfalls resulting in small-scale advances of glaciers in these regions. This accounts for the variability in timing and extent of these glacial advances, while the resulting increased Chinook activity produced the development of a sand sea between Medicine Hat and Regina on the southern Prairies. The cT air mass was relatively weak, permitting these changes. During the maximum of the Altithermal/Hysithermal warm event （6,000 years B.P.）, the Arctic Front had retreated into the southern Yukon Territory as the cT air mass became stronger. The mP air could not move inland as easily, resulting in drier climates across the region. Prairie plants mi- grated into the southern Yukon Territory, and land snails from the eastern United States were able to migrate up the Saskatchewan River system as far as Lake Louise, Alberta. On the southern Prairies, the many small sloughs and lakes dried up. During the maximum of the Late Wisconsin Glacial event （15,000 years B.P.）, the Arctic Front had moved south to the vicinity of 30°N, while there had been a southward movement of the Zone of Intertropical Convergence from the equator to about 10°S. The mP air was also very strong and dumped enormous quantities of snow in the glaciated Canadian Cordillera, but it does not appear to have moved south any distance into the northern United States, witness the limited glaciation and widespread permafrost that developed there. Instead, there is evidence for buffering of the climatic changes in the closed basins in the northern Cordillera of the contiguous United States. The source of the cT air mass had moved south into the northern part of South America, permitting an exchange of savannah biota between the two continents. An extensive area of white dune sands inundated both savannah and forest along the inland hills in Guyana. This parallels the massive changes in African climatology during the last Ice Age （Fairbridge, 1964）. If these changes occurred each time there was a major glaciation in the Northern Hemisphere, this would explain the movement of biota from all terrestrial environments between the two American continents in the last 2 million years. A similar northward movement of climatic belts occurred in South America, with the cA air from Antarctica expanding northwards into southern Argentina and Chili. However paucity of data and the potential effects of El Ni o and the Southern Oscillation make it difficult toprovide details of the changes there in the present state of knowledge. This technique of studying the mechanisms of present-day climatic changes and applying the results to past climatic events has considerable potential for elucidating past climatic changes elsewhere in continental regions. This may prove particularly valuable in studying the Siberian anticyclone that is the main cause of the distribution of permafrost, but this will need international cooperation to be successful. 相似文献
35.
人类已经叩响了区块链时代的大门,但在地质领域,区块链的应用几乎还处于空白的状态。哪怕是像深时数字地球(DDE)这样极具雄心的大科学项目,也没有意识到区块链技术有助于分散在科学家个人或实验室的“长尾数据”的有效收集和利用。地质科学信息是一个超大型共享数据库,需要许多人彼此信任去协作完成数据库的改写,区块链将是非常好的支撑技术。区块链基于其系统特性和智能合约提供有丰富交互接口,特别是其具有去中心化、不可篡改、隐私保护特性,为区块链技术在地质领域应用提供了重要的基础。地质勘查实物、资料、数据的溯源管理,面向更广泛科学共同体的长尾数据的收集,应该是区块链地质应用的重要突破口。地质通证使打造全球地质社区成为可能。 相似文献
36.
北京朝阳门地区一件不完整的下凳骨化石，代表了中更新世中期至晚更新世早期德氏水牛（Bubalus teilhardi)在北京城区的首次发现，具有重要的地层学，古气候学意义，该化石的发现表明，北京城区在十几万年前气候较为温暖湿润。 相似文献
37.
A. F. Trendall 《Australian Journal of Earth Sciences》2013,60(3):287-311
38.
Potential of submarine-cave sediments and oxygen isotope composition of cavernicolous micro-bivalve as a late Holocene paleoenvironmental record 总被引：2，自引：1，他引：1
Akihisa Kitamura Nagisa Yamamoto Tomoki Kase Shu-ichi Ohashi Mayumi Hiramoto Hitoshi Fukusawa Tsuyoshi Watanabe Tomohisa Irino Hiroshi Kojitani Michiyo Shimamura Ikuo Kawakami 《Global and Planetary Change》2007,55(4):301-316
A sediment layer (43 cm thick) and surface sediments (5 cm thick) in a submarine limestone cave (31 m water depth) on the fore-reef slope of Ie Island, off Okinawa mainland, Japan, were examined by visual, mineralogical and geochemical means. Oxygen isotope analysis was performed on the cavernicolous micro-bivalve Carditella iejimensis from both cored sediments and surface sediments, and the water temperature within the cave was recorded for nearly one year. These data show that: (1) water temperature within the cave is equal to that at 30 m deep in the open sea; (2) the biotic and non-biotic environments within the cave have persisted for the past 2000 years; (3) mud-size carbonate detritus is a major constituent of the submarine-cave deposit, and may have come mainly from the suspended carbonate mud produced on the emergent Holocene reef flat over the past two millennia; (4) the δ18O-derived temperature (Tδ18O) of C. iejimensis suggests that the species grows between April and July; (5) the Tδ18O of C. iejimensis from cored sediments implies that there were two warmer intervals, at AD 340 ± 40 and AD 1000 ± 40, which correspond to the Roman Warm Period and Medieval Warm Period, respectively. These suggest that submarine-cave sediments provide unique information for Holocene reef development. In addition, oxygen isotope records of cavernicolous C. iejimensis are a useful tool to reconstruct century-scale climatic variability for the Okinawa Islands during the Holocene. 相似文献
39.
Tomi P. Luoto 《第四纪科学杂志》2009,24(2):150-158
Numerical techniques were used to study chironomid distribution and abundance in lakes from a 1000 km transect in Finland, with special interest on the effect of local summer air temperatures on chironomid assemblages. The final aim of the study was to develop a chironomid‐based palaeotemperature inference model. The dataset consisted of 82 lakes (of which 77 were used in the model after deletion of outliers), with catchments spanning from boreal coniferous forests to mountain birch woodland and tundra vegetation. Numerical analysis showed that the mean July air temperature was the most significant variable explaining the distribution and abundance of chironomids in Finnish lakes. Weighted‐averaging partial least squares techniques were used to develop a palaeotemperature inference model for mean July air temperature reconstructions. The model performance statistics were favourable, with cross‐validated coefficient of determination (r2) of 0.78, root mean squared error of prediction of 0.721°C and maximum bias of 0.794°C. Based on these values, the transfer function is a valid means of performing quantitative palaeotemperature estimates in downcore studies. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
40.
MAXIM OGURTSOV HOGNE JUNGNER MARKUS LINDHOLM 《Geografiska Annaler: Series A, Physical Geography》2007,89(2):129-136
Six millennial proxy records of temperature in the northern hemisphere were analysed using both the Fourier and wavelet approaches. We found that the analysed temperature proxies have appreciable synchrony at multidecadal and centennial time scales. These data also show evidence for the presence of a roughly regular large‐scale rhythm with a periodicity of 50–130 years in the climate of the northern hemisphere over the last millennium. It is shown that the amplitude of this variation might reach 0.20–0.28°C and contribute appreciably to the rise of global temperature over the first part of the 20th century. Possible origins of the global centennial climatic cycles are discussed. 相似文献